1. Field of the Invention
The present invention is directed to an X-ray diagnostic facility with an X-ray device for generating X-radiation of the type having a digital X-ray detector for acquiring the X-rays and converting them into an electrical signal sequence that has picture elements arranged in a first structure, and having an image system for processing the electrical signal sequence as well as a playback device, and having a stray radiation grid having a second structure of highly absorbent material and highly X-ray transparent material in alternation, preceding the X-ray detector.
2. Description of the Prior Art
Digital X-ray detectors have been changing classic radiography and angiography and cardangiography for years. A wide variety of technologies have already been in use for some time or are about ready for the market namely digital technologies such as image intensifier camera systems based on video or CCD camera, storage film systems with an integrated or external readout unit, systems with optical coupling of the converter foil to CCDs or CMOS chips, selenium-based detectors with electrostatic readout (for example, Thoravision) and solid-state detectors with active readout matrices.
In particular, new solid-state detectors for digital X-ray imaging have been under development for a few years that are based on active readout matrices, for example composed of amorphous silicon (a-Si). The image information is converted in an X-ray converter, for example cesium iodide (CsI), stored in the photodiodes of the matrix as electrical charge, and subsequently read out via an active switch element with dedicated electronics and analog-to-digital converted and further-processed by the image system.
Related technologies likewise employ an active readout matrix of amorphous silicon but a converter (for example, selenium) that directly generates electrical charge that is then stored on an electrode. Other technologies are based on CCDs (charge coupled devices) or APS (active pixel sensor) or CMOS chips.
All of the above-described technologies ultimately make the X-ray information available in digital form, both in terms of position by means of a 2-dimensional pixel structure as well as in terms of the signal amplitude by means of gray values having a given bit depth (length).
For suppressing stray radiation that arises in the examination subject, a stray radiation grid as disclosed, for example, by European Application 1 126 289 can be utilized for improving the image quality in the majority of all examinations. All conventional stray radiation grids are composed of a regular structure of highly absorbent material such as, for example, lead and high X-ray-transparent material such as, for example, paper, in alternation.
Aliasing effects arise due to the combination of two components with regular structures in the position domain—digital X-ray detector with pixel structures, as one component, and a stray radiation grid with absorber lamellae, as the other component. This is particularly the case when the digital X-ray detector has a high modulation transfer function (MTF) in the region of the spatial frequency of the stray radiation grid. These aliasing effects can generate considerable artifacts and overlay diagnostically relevant information.
This effects will play an increasingly greater part in the future due to the introduction of technologies with higher resolution, for example due to smaller pixels, optimized X-ray absorption and/or direct converter materials.
Stray radiation grids have been employed conventionally as moving grids or stationary multi-line grids for individual frames. Stationary grids are employed in angiography and fluoroscopy with high image frequency.
U.S. Pat. Nos. 6,366,643 and 5,949,850 disclose stray radiation grids with regularly repeating, irregular structures. Two equidistant geometrical structures are placed on top of one another.